The present invention is related generally to machine manufacturing of components. In particular, the present invention is related to rapid prototyping manufacturing including layered manufacturing and solid freeform fabrication.
Using conventional techniques, a desired article to be made can initially be drawn, either manually or automatically utilizing a computer-aided design (CAD) software package. The article can be formed by removing material from material stock to form the desired shape in a machining operation. The machining operation may be automated with a computer-aided machining (CAM) process. The design and manufacture process may be repeated multiple times to obtain the desired part. A common practice is to mechanically remove material to create three-dimensional objects, which can involve significant machining skills and turn around time.
One process for making three-dimensional objects builds up material in a pattern as required by the article to be formed. Masters, in U.S. Pat. No. 4,665,492, discusses a process in which a stream of particles is ejected and directed to coordinates of the three-dimensional article according to data provided from a CAD system. The particles impinge upon and adhere to each other in a controlled environment so as to build up the desired article.
Processes and apparatus also exist for producing three-dimensional objects through the formation of successive laminae which correspond to adjacent cross-sectional layers of the object to be formed. Some stereo lithography techniques of this type use of a vat of liquid photocurable polymer which changes from a liquid to a solid in the presence of light. A beam of ultraviolet light (UV) is directed to the surface of the liquid by a laser beam which is moved across the liquid surface in a single plane, in a predetermined XY pattern, which may be computer generated by a CAD system. In such a process, the successive layers may be formed in a single, horizontal plane, with successive layers solidifying together to form the desired object. See, for example, U.S. Pat. No. 4,575,330 to Hull. Arcella et al., in U.S. Pat. No. 4,818,562, discuss a method for forming an article by directing a laser beam on a fusible powder which is melted by the beam, and which solidifies to form the desired shaped object.
Recently, various solid freeform fabrication techniques have been developed for producing three-dimensional articles. One such technique, used by Stratasys, Inc. (Eden Prairie, Minn.), is referred to as Fused Deposition Modeling (FDM). See, for example, U.S. Pat. No. 5,121,329 to Crump, herein incorporated by reference. FDM builds solid objects, layer by layer, from polymer/wax compositions according to instructions from a computer-aided design (CAD) software program. In one FDM technique, a flexible filament of the polymer/wax composition is heated, melted, and extruded from the nozzle, where it is deposited on a workpiece or platform positioned in close proximity to the dispensing head. The CAD software controls the movement of the dispensing head in the horizontal X-Y plane and controls the movement of the build platform in the vertical Z direction. By controlling the processing variables, the extruded bead or xe2x80x9croadxe2x80x9d can be deposited layer by layer in areas defined by the CAD model, leading to the creation of the desired three-dimensional object. Other examples of layered manufacturing techniques include multi-phase jet solidification techniques and/or laser-engineered net shaping. The extruded bead can be a ceramic suspension or slurry, a molten plastic, a powder-binder mixture, a polymeric material ready for curing or hardening, a molten metal, or other suitable materials which harden with time and/or exposure to an external stimulus. The bead can also be a curable strip of polymer or pre-polymer with polymerization initiated by radiation.
In conventional layered manufacturing techniques, the layers are formed or deposited in a flowable state which can be in the form of a series of long beads of extruded material. The beads can have a rounded, oblong, or circular transverse cross-sectional profile, where the external side faces of the bead can bulge outward. The conventional material layers are typically rounded at the periphery, forming layer surfaces having convex intra-layer regions and sharp, mechanically weak concave inter-layer regions. In particular, where the stacked bonded layers form the manufactured part side surfaces, the concavities can form sharp crevices having poor properties with respect to crack propagation and fracture.
In conventional layered manufacturing, cavities, either external or internal, are often found in product designs. The cavities may have upper structures such as ceilings or overhangs. The upper structures may be cantilevered structures having one end or edge free or structures only unsupported in the middle, between supports on either side or edge. The structures are unsupported in the sense that during deposition or formation of the still flowable main material, the material will drop down through the cavity without a structure previously established to support the main material during hardening. The cavities below have a volume which can be defined by a downward projection of the unsupported portion of the main material above.
In conventional layered manufacturing, a support structure of secondary material is built, layer by layer, to provide a support structure for the material to be formed or deposited in the layer above. The secondary material forms layers which also require support from the layer below for their deposition. Using conventional methods, an unsupported structure is supported by secondary material, layer under layer, from top to bottom, until the bottom of the cavity is reached, or until the workpiece platform being used to build the article is reached. The secondary material is later removed by mechanical, chemical, or thermal means, leaving the main material article. A large amount of secondary material can be required to build the removable structure, as well as a large build time required to form the secondary material layers.
What would be desirable are methods suitable for making parts using layered manufacturing which provide superior crack resistant surfaces. Methods which require less time to build support structures would also be advantageous.
The present invention includes improved methods for making objects using layered manufacturing techniques, as well as the objects made possible through use of these methods. One group of methods forms objects having improved surface properties made possible by forming a mold layer of a second material prior to forming a main part layer of a first material. Another group of methods forms objects requiring less time and material to build. This group of methods includes methods for building minimized secondary material support structures having less volume than conventional support structures.
More particularly, the present invention includes methods for forming a mold layer of a second material along the periphery of the object surfaces to be improved. The second material layers can be convexly rounded at the periphery, forming a rounded mold layer to receive the later formed first material. The first material layer can thus form an impression of the second material layer along the periphery of the first material layer. The impression formed along the first layer side face can have a rounded, concave, middle intra-layer region and a convex, inter-layer region where the multiple layers stack together. The inter-layer convexities have superior mechanical strength and superior crack resistance relative to the concave inter-layer regions of the conventionally made parts.
In one method, a data file containing representations of a three-dimensional object is accepted as input. The data file can be a three-dimensional CAD file, for example, a stereo lithographic (STL) file. The three-dimensional data can be partitioned into horizontal slices or layers, which can be represented by two-dimensional closed curves or poly-line segments having an associated layer thickness. The curves can define the outside and/or inside of areas to be filled with the main material. The curves can later be filled with raster tool paths generated to fill the area with material. The user can identify surfaces of the three-dimensional object to receive surface improvement and, directly or indirectly, identify the curves or curve portions corresponding to the surfaces to be improved.
A set of secondary curves can then be generated, the secondary curves corresponding to secondary material areas to abut the main material areas. The secondary curves thus formed preferably correspond to layer areas having at least two bead widths of secondary material. Some embodiments form secondary material layers with no voids, while other embodiments form secondary material layers having voids to reduce material usage and build time. The secondary material curves can then be used to generate tool paths for the secondary material. The secondary and main material tool paths can be checked for consistency and lack of interference before being integrated and the processing completed.
In the manufacturing phase, the part can be built up, bottom to top, by depositing the secondary and main materials, layer by layer. If secondary material is called for in the current layer, a secondary material nozzle can deposit a bead of secondary material of the desired bead width along the previously calculated path. A main material nozzle can then deposit a bead of main material of the desired bead width and along the previously calculated tool path. The flowable main material, formed along the previously formed secondary mold layers, can form an impression of the mold layers convex edge shape, thereby attaining a concave intra-layer shape and a convex inter-layer shape, where the stacked layers join each other. The secondary material can be later removed, exploiting differential mechanical, chemical, or thermal properties. In a preferred embodiment, the main and secondary materials are not the same, but are the same material in other embodiments. Improved surfaces provided by the present invention can have improved mechanical properties due to the lack of sharp, inter-layer convexities.
The present invention also includes methods for building removable support structures that form the secondary structures using substantially less volume than the cavity volume. The support structures can have at least one sloping side surface having a substantial deviation from vertical. In one group of structures, the support forms an angle or corner brace, supporting the cavity ceiling from a side wall. The angle piece can have a width decreasing with depth, indenting or offsetting until the support piece has no width. In another group of structures, the support forms a column or interior wall having a wide topmost layer and less wide middle and bottom layers. The wide top layers support the main material layer above, with the lower layers decreasing in width. The lower layers can be indented or offset inward by a small amount at each layer. The indent amount is preferably less than about one-half of the bead width of the layer above.
One method for generating the minimized support structures accepts two-dimensional curves for each layer as input. The two-dimensional curves represent the inner and outer perimeters of the main material layers for the part to be built. The unsupported or overhanging structures can be identified by processing the layers of the main structure from top to bottom, beginning with the second to top layer. The layers can be processed as pairs having an upper and lower layer. The upper layer can be reduced in one or more dimensions by an indent or offset amount ultimately corresponding to the slope of the side surface of the minimized support structure. In some embodiments, certain dimensions are automatically or manually selected as not to be reduced in extent. The difference of the reduced projected upper layer and the lower layer corresponds to an unsupported upper area, which will require support prior to formation. New secondary support material curves can be generated at the current lower level to provide the missing support, and these newly added secondary support material curves added to the main material curves for the current, lower layer. The newly added curves will also require support from below during formation, and are added to the set of main material curves, but are identified as secondary material curves.
The current layer can be set to be the next lower layer, making the previous lower layer of the pair the upper layer, and the process repeated. The new calculation will now take into account any curves representing either unsupported main material or secondary support material. The process can be repeated for all layers of the part to be made.
One output of the method can be a set of secondary material curves to be filled with secondary support material. The secondary material curves can be further processed by raster filling the areas within the curves using conventional rasterizing techniques. The curves and tool paths generated can be checked for consistency and lack of interference, both within the secondary material and between the secondary and main materials. The rasters can be used as tool paths to control the formation or deposition of main and secondary material.
In manufacture, the main and secondary material tool paths can be fed to a layered manufacturing machine for each layer. The minimized support sloping side faces, which were likely calculated top down, are built bottom up. The sloping side faces of the support structures can be built with a slight overhang at each higher level, the overhang preferably not exceeding one-half (xc2xd) a bead width. The secondary material support structures can thus be built to have large dimensions at the topmost layer. In some objects, the next layer up will consist of a main material layer deposited on the now solidified secondary material layer.